Your question is NULLs in Aggregate Metrics. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Aggregate calculations are common in reporting and data quality checks, and NULL handling often changes the meaning of a result. Interviewers ask this to confirm that you understand both SQL behavior and business interpretation.
Explain how PostgreSQL aggregate functions such as SUM() and AVG() handle NULL values. You should describe what happens when some rows are NULL, what happens when all rows are NULL, and when you would use COALESCE() to replace NULL values before or after aggregation.
Keep your answer practical. Focus on SUM(), AVG(), COUNT(column), and COUNT(*), and explain the difference between ignoring missing values versus treating them as zero in a reporting context. A short PostgreSQL example is enough, but the interviewer will expect you to call out common mistakes.